Recently literature reports, such as J. Th. W. de Hair, "The intermediate role of Gd.sup.3+ in the energy transfer from a sensitizer to an activator", Journal of Luminescence 18/19, (1979) 797-800, J. Th. W. de Hair and W. L. Konijnendijk, "The intermediate role of Gd.sup.3+ in the energy transfer from a sensitizer to an activator (especially Tb.sup.3+)", Journal of Electrochem Society, Vol. 127, #1, (1980) 161-164, T. E. Peters, R. G. Pappalardo, and R. B. Hunt Jr., "Unusual green emission from Mn.sup.2+ and Gd(BO.sub.2).sub.3 ", Journal of Luminescence 31 and 32 (1984 290-292), have described how electronic excitation-energy is efficiently transferred in oxide hosts from Gd.sup.3+ centers to coactivators capable of visible emission, such as Tb.sup.3+, Dy.sup.3+ and Mn.sup.2+. However, there is no mention of efficiently transferring non-radiatively excitation energy absorbed by Gd.sup.3+ centers to minority Er.sup.3+ centers, whose emission produces green light as disclosed in the present invention.